l2arc - why not attempt to make it persistent?

Marty Rosenberg marty.rosenberg at gmail.com
Mon May 23 16:22:09 EDT 2011


what happens when the cache device is actively wrong? for example, you have
a zpool,
with a cache device, unplug it for a few years, then plug it back in? I know
that when
a cache device is failing, it will be ignored, but I do not remember seeing
any assumptions
being made about the data on the cache device simply not reflecting the data
in the zpool
--Marty

On Mon, May 23, 2011 at 12:51 PM, Brian Behlendorf <behlendorf1 at llnl.gov>wrote:

> This is a good idea which has come up a few times.  Sun/Oracle has
> mentioned previously that it's a feature they would like to see added.
> Unfortunately, it is feature which would primarily benefit desktops and
> laptops which reboot often.  It's less of an issue for servers which
> reboot rarely and should usually have a hot cache.  I think it's pretty
> clear that features which benefit servers have historically gotten the
> most development effort.
>
> However, it remains a good idea!  We can certainly add it to our feature
> wish list, and perhaps someone will decide to work on it.
>
> --
> Thanks,
> Brian
>
> On Sun, 2011-05-22 at 08:05 -0700, Uncle Stoatwarbler wrote:
> > Given that:
> >
> > 1: If there is a cache device problem the cache device gets dumped.
> > 2: If there is a cache object inconsistency with what's on disk then the
> > cached objects get dumped.
> > 3: if there are any other kinds of inconsistencies then the disk wins
> >
> > Why not attempt to preserve the l2arc cache across reboots/mounts and as
> > an effect, (hopefully) speed up access from the outset rather than
> > having to wait for the cache to build out again? (if there are any
> > problems, the cache should be dumped, but if it's intact, why dump it?)
> >
> > I've just been benchmarking my (rather slow, cheap) ssds vs disks and
> > even for the worst case scenarios there are 5-10 times the speed of the
> > spinning media for all except sequential writes (where they are only
> > twice the speed)
> >
> > My concern partly stems from the limited write cycles of ssds
> > (especially cheap ones) and having just seen 40Gb of cache (on a 64Gb
> > cache device) being dumped, but also from the realization that my system
> > is significantly slower after a reboot, until the cache is repopulated
> > (zfs on root)
> >
> > Alan
> >
>
>
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